The present invention relates to a method of receiving a signal transmitted by orthogonal frequency-division multiplexing (OFDM), in which the multiplexed subcarrier signals are modulated by differential phase-shift keying, and to a digital receiver employing this method.
OFDM is particularly advantageous for broadcasting signals to mobile receivers, which must contend with such problems as multipath fading. Recommendation BS.774, drafted by the Radiotelecommunication Standardization Sector of the International Telecommunication Union (ITU-R), proposes a digital audio broadcasting standard, referred to below as DAB, in which OFDM is employed with differential phase-shift keying.
The DAB subcarrier signals are separated from one another by a constant frequency spacing, the modulated subcarriers being positioned above and below a central subcarrier which is not modulated. A conventional receiver down-converts the received signal by mixing it with the output of a local oscillator tuned in a precise frequency relationship to the central subcarrier, performs a complex Fourier transform to obtain phase information for each modulated subcarrier, then performs differential demodulation and further decoding to obtain the broadcast audio data.
A problem with this receiving method is that the local oscillator must be tunable over a comparatively wide frequency range. Local oscillators with wide tuning ranges are costly, and are also difficult to tune accurately. The conventional receiving method therefore leads to an expensive receiver with undesirable tuning error.
A simple solution to this problem would be to use a less expensive and more accurate local oscillator having a narrower tuning range, and tune the local oscillator in relation to any one of the subcarriers. If the local oscillator is not tuned in relation to the central subcarrier, the array of differentially demodulated subcarrier data will be offset by a certain number of subcarriers, but the elements in the array can be shifted in the opposite direction by the same number of subcarriers to compensate for the offset. Unfortunately, the data symbols in the DAB signal are separated by guard intervals, to prevent inter-symbol interference, and these guard intervals produce a type of error that cannot be corrected by a simple array shift. Details will be given later.